Techniques for controlling the set point of a property of a material being formed in response to an indication of a spread of the property values are known; see U.S. Pat. Nos. 3,515,860, Fitzgerald Jr., and 3,622,448, Adams et al, 3,648,035, Hart et al, and the copending application, commonly assigned with the present application, of David A. Spitz, Ser. No. 189,252, filed Oct. 14, 1971.
In systems of this type, the spread of values of the property is indicated in response to computation of the standard deviation of the property or the percent of the property which is considered defective; i.e., the percent of the property having a value outside of a limit value. By controlling the set point of the property in response to the spread of values, the average value of the property is adjusted so that the amount of raw material used to form the finished product is minimized.
The prior art techniques dealt with finished products wherein the properties were independent and did not interact with each other. Hence, controlling the set point of one property could be performed independently of control for the set point of another property, even though the two properties were on the same finished product. For example, in the Adams et al patent, moisture and bone dry basis weight set points of a paper sheet being manufactured are controlled to reduce the amount of fiber required to form the sheet by minimizing the bone dry basis weight set point and maximizing moisture to produce a sheet having certain percentages of excessively low (defective) bone dry basis weight and excessively high (defective) moisture. The moisture and bone dry basis weight set points are controlled completely independently of each other.